1. Field of the Invention
The present invention relates to a system for controlling a driving force in a hybrid vehicle which is provided as its prime mover with an internal combustion engine such as a gasoline engine or a Diesel engine and an electric motor such as a motor/generator for outputting a torque when energized by an electric power.
2. Related Art
The hybrid vehicle is a vehicle which has been developed with a view to improving the fuel economy and reducing the exhaust gasses, and has an internal combustion engine, an electric motor and a power generator mounted thereon. For a run, moreover, the hybrid vehicle is controlled to select a drive mode (or running mode) in which the fuel consumption is minimized.
Although the fuel is naturally consumed when the vehicle is run with the internal combustion engine, the electric power, as generated with the power generator by running the internal combustion engine, is consumed when the vehicle is run with the electric motor. After all, the fuel is indirectly consumed even if the vehicle is run with the electric motor. In other words, the effect of improving the fuel economy may differ depending upon the situations even when the vehicle is run with the electric motor.
When a high motive power is required at a low vehicle speed, the fuel consumption in the internal combustion engine increases to deteriorate both the fuel economy and the exhaust gasses. Similar situations are invited even while the vehicle is running, at the time of an acceleration when the degree of accelerator opening is large. Generally in the prior art, therefore, at a start or acceleration, the vehicle is run either with the electric motor or by adding the torque of the electric motor to the torque of the internal combustion engine. When the vehicle is run at a constant or higher speed, only the internal combustion engine is activated. Simultaneously with this, the electric power is generated at a low SOC (i.e., State Of Charge: charge state in a battery device) to charge the battery by the internal combustion engine.
In recent years, on the other hand, there has also been developed a system for selecting a drive source allowing the best fuel economy for generating a driving force demanded, in place of the aforementioned concept in which the running mode is changed depending exclusively upon the vehicle speed and the accelerator opening. This system is disclosed in Japanese Patent Laid-Open No. 9-98516 (JPA9-98516).
The disclosed system has contemplated to select a running mode for optimizing the fuel consumption, by considering the driving efficiency and the power generating efficiency, when the fuel consumption at the time of running the vehicle by generating the demanded driving force in the internal combustion engine and the fuel consumption necessary for the electric power generation are to be determined, and by considering an efficiency at the time of running the vehicle with the electric motor. When much regenerative energy is generated at a deceleration, moreover, the running region with the electric motor is widened.
The system disclosed in the Laid-Open is characterized in that the efficiency such as the driving efficiency or the charging efficiency is taken into consideration when either the internal combustion engine or the electric motor is to be selected as a drive sorce to minimize the fuel consumption for generating the demanded motive power. It can be incorporated into the control that those efficiencies are not necessarily constant but different according to the running state of the vehicle or the charged state of the battery device. This makes it possible to improve the fuel consumption and the exhaust gasses better.
Here, the efficiency such as the driving efficiency or the charging efficiency accompanying the change in the motive power source differs depending upon the state of the vehicle, as described in the above-specified Laid-Open. In the hybrid vehicle, in which the charging operation is performed while it is running, the fuel consumption necessary for the charge or the fuel consumption per unit of the electric power reserved already may be different. Specifically, there may be the charging cases consuming the higher and lower fuel consumption.
In the invention disclosed in the Laid-Open, the amount of fuel to be consumed for generating the demanded motive power and the amount of fuel to be consumed for achieving the electric power for generating the motive power are compared to select the running mode, i.e., the motive power source of the less fuel consumption. However, the disclosure has failed to consider how much fuel has been consumed for the reserved electric power. Specifically, the electric powers, as reserved by consuming the more and less fuels, are equivalently handled so that no consideration is taken into the so-called "unit price" of the reserved electric power (i.e., the fuel consumption per unit of the reserved electric power). This may admit use of an electric power of high unit price thereby to cause situations such as a reduction in the effect of improving the fuel economy.